Oxygen, determination sensor

Table 1. Microbial species used for BOD-sensors based on amperometric oxygen determination and their parameters (it is noted when other transducer are used)... 

Figure 19.6— The Clark sensor for oxygen determination. The Teflon membrane, permeable to gas, must be very close to the cathode so that the double diffusion process through the membrane and the liquid film will lead to a stable signal within a few seconds.

The possibility of determining other gas constituents besides oxygen using sensors with oxide-ion-conducting solid electrolytes arises from the thermodynamic equilibria that are set up with oxygen in the gas phase or between particles in the gas phase and the solid electrolyte. 

Only for particular molecules, e.g. ammonia because of its strong lines in the 20-40 GHz region, or water at 22 GHz because there is no other line until 183 GHz, would spectral considerations force the worker to lower frequencies. The 20-40 GHz band is also attractive, however, because of the cheap sources and low-noise semiconductor detectors, manufactured for movement sensors and short-path wireless links. The projected automobile collision-avoidance radar systems will make cheaper sources and detectors available for the 60-70 GHz region within the next few years. The 60 GHz across-office circuits for wireless data links could provide useful narrow-band sources for oxygen determination. The 35 GHz and 94 GHz close-range radar bands provide a useful reservoir of components and sources for the potential manufacturer of MMW spectrometers. 

The Clark electrodes described above are not suitable for oxygen determination in dry gaseous samples such as air because the thin layer of electrolyte solution contained behind the membrane is prone to rapid drying. A different arrangement is therefore used for such applications. Amperometric gas sensors for oxygen (and sensors for other electroactive species in the vapor phase) usually consist of a porous PTFE membrane that bears a precious metal electrode deposited, also in porous form, directly on the backside. This keeps the diffusion length short while... 

For oxygen determinations within the framework of WOCE (World Ocean Circulation Experiment) an algorithm to convert the CTD oxygen sensor measurements into oxygen profiles based on the documented sensor physics and in situ oxygen data of discrete samples is described in the WOCE Opierations Manual (WHP, 1994). The achievable accuracy and precision is < 1 % and 0.1 %, respjectively. 

For the measurement of gas components like hydrocarbons (HC) or nitric oxides (NOx) in non-equilibrated gas phases kinetically determined sensors are used (Fig. 19.2 middle).Depending on the electrode material, the gas components do not equilibrate on the measuring electrode at temperatures <700 °C. Thus gas components which are not thermodynamically stable are electrochemically active. In an HC- and 02-containing gas, for example, at least two electrode reactions can take place the electrochemical reduction of oxygen and the electrochemical oxidation of hydrocarbons. The measured open-circuit voltage does not obey the Nemst equation. Therefore such electrode behaviour is often referred to non-Nernstian electrodes (or mixed potential sensors). The cell voltage depends logarithmically on the concentrations of the hydrocarbons ... 

To determine water temperature, conductivity, pH, and the concentration and saturation of oxygen, electronic sensors were used in the field. The WTW LF 196, pH 196, and 196 devices were employed for these analyses. The measurement of pH in water with an electric conductivity as low as that in the Rio das Mortes is hi y problematic, and the values recorded contribute little to an understanding of the water chemistry. The electronic sensor for pH had to be left in the water for 30 to 45 minutes to permit it to stabilize. 

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

Air is normally the reference gas used in the exhaust gas sensor. If the oxygen partial pressure in the engine exhaust gas is known as a function of the engine air/fuel ratio, the theoretical galvanic potential of the sensor is easily determined by the Nernst equation. 

Sensor-based methods. Whilst many methods use sensors, the simplest being temperature measurement, this terminology is often used to cover viscosity, pH, oxygen and humidity determination, etc. These are true in-line techniques and offer rapid, inexpensive real-time analysis. Humidity determination in drying ovens is a common example. 

An example of amperometric methods used for analytical purposes is the sensor proposed in 1953 by Leland C. Clark, Jr. for determining the concentration of dissolved molecular oxygen in aqueous solutions (chiefly biological fluids). A schematic of the sensor is shown in Fig. 23.1. A cylindrical cap (1) houses the platinum or other indicator electrode (2), the cylindrical auxiliary electrode (3), and an electrolyte (e.g., KCl) solution (4). The internal solution is separated by the polymer... 

---